It is important in an optical communication system using optical fibers to increase the receiving sensitivity and the frequency utilization efficiency per optical fiber in order to achieve a long-distance and large-capacity communication. It has been proposed to switch various modulation schemes that differ in the reachable transmission distance and the frequency utilization efficiency depending on a transmission distance and a transmission capacity that are required because the optical communication has a trade-off relationship between increase in transmission distance and improvement in frequency utilization efficiency. Examples of modulation schemes that differ in the reachable transmission distance and the frequency utilization efficiency include BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), 8 QAM (quadrature amplitude modulation), and 16 QAM.
One example of optical transmitters that are used switching modulation schemes is described in Patent Literature 1. The related optical transmitter described in Patent Literature 1 includes a client accommodation unit, a variable frame mapping unit, a variable coder, an optical modulation unit, and a transmission method setting unit.
The client accommodation unit terminates a client signal transmitted from a client. The variable frame mapping unit allocates the client signal terminated by the client accommodation unit to a predetermined transmission frame. At this time, the variable frame mapping unit performs mapping in accordance with a transmission method selected by the transmission method setting unit.
The variable coder generates a modulation signal used for carrying the transmission frame generated by the variable frame mapping unit. At this time, the variable coder generates the modulation signal in accordance with the transmission method selected by the transmission method setting unit. The optical modulation unit generates a modulated optical signal from the modulation signal generated by the variable coder and outputs the modulated optical signal. At this time, the optical modulation unit generates the modulated optical signal in accordance with the transmission method selected by the transmission method setting unit.
The transmission method setting unit selects a transmission method corresponding to a transmission rate of the client signal from among a plurality of transmission methods provided by the optical transmitter. In addition, the transmission method setting unit notifies the variable frame mapping unit, the variable coder, and the optical modulation unit of transmission method information indicating the selected transmission method.
As mentioned above, the related optical transmitter is configured to transmit a client signal using the transmission method corresponding to the transmission rate of the client signal. It is said that this configuration enables the transmission efficiency to be improved because the transmission amount of a useless signal is small even though the transmission rate of the client signal is low.
Other related techniques are described in Patent Literature 2 to Patent Literature 4.